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Technologies For Grey Water Management at Household Level

(1) Leach Pit



      This is the simple and most cost effective option for the treatment of household grey water. Besides, requiring minimum space operation and maintenance (O&M) of this system is also minimal and easy.


Description


      This is a brick-lined pit constructed in the courtyard of a house at a convenient place. The grey water from the house (kitchen wastewater, bathing water, washing water, etc.) should be directed to this pit. It is essential to pass the water through a nhani trap or P trap to avoid entry of mosquitoes and exit of foul odour. A silt chamber, prior to the leach pit, retains solids and ensures proper functioning of the leach pit. The pit is suitably covered with flag stones or Reinforced Concrete Cement (RCC) cover of required dimensions.


Constructed Leach Pit

Leach Pit under construction

Periodical de-silting of Leach Pit

Significance of a Leach Pit


  • Dry environment: the entire wastewater from a household is absorbed by this underground structure. This results in dry premises;
  • Freedom from the mosquito nuisance;
  • Odour-free environment;
  • No drains required;
  • Recharge of ground water: the wastewater enters the natural ecosystem and finally reaches to the ground water stream thus recharging the ground water source; and
  • O&M costs are low and borne by the house owner.

Site Selection


  • The pit should be located 1 m (3 feet) away from the house wall;
  • It should be 3 m (10 ft) away from any ground water source; and
  • It should be as near as possible from the source of wastewater (to save on the pipeline).

Design Considerations (Refer Figures 9 and 10)


  • Shape: preferably circular (cylindrical) to reduce cost, yet durable and maximizes volume;
  • Bottom of the pit: not to be cemented or concreted to facilitate seepage of water;
  • Construction: First layer should be 225 mm (9” brickwork). All other layers above in 110 mm (4.5” brickwork) These layers should have honeycomb masonry in alternate layers to facilitate seepage of water;
  • Inlet pipe: 63 to 75 millimeter (mm) diameter PVC (Polyvinyl Chloride) or SWR (Soil, Waste and Rain) pipe: to be connected to the pit keeping 200-225 mm freeboard;
  • Volume: Effective volume (volume up to pipe level) should be kept equivalent to double the daily discharge of grey water from the house. Generally, it should be 0.82 cum (28 cft). The volume can be increased if the wastewater generation is higher.
  • Alternate construction material: the pit can be conveniently constructed using prefab cement rings with holes and an RCC or ferrocement cover.

Step-by-step Construction (Refer Figures 9 and 10)


  1. Excavation Dig a pit with 1.5 meter diameter and 1.25 m depth
  2. Construction Construct the pit in single brick with honeycombing in alternate layers. Honeycombing should be through 6 to 8 holes of 40 mm each. Brick masonry is to be done in 1:6 cement mortar
  3. Connect the pipe at a height of 1 m from the bottom. The pipe should protrude 100 mm inside the pit.
  4. Cover: 40-50 mm thick RCC cover cast in two halves. Alternatively, readymade RCC covers or ferrocement slabs or good quality flag stone or Cuddappa stones can also be used.
  5. Cover to be properly sealed to avoid vector entry
  6. Cover to be covered with soil layer of about 200 mm

Connection with Household Wastewater


  1. As described in previous paragraphs, the household wastewater must pass through a nhani trap or a P trap to avoid vector entry and odour nuisance. The nhani trap should also be covered with a suitable screen to screen out solids from the wastewater.
  2. If excess amount of impurities such as ash/dirt are expected, a silt chamber (Refer Figures 9 and 10)at a suitable point is advisable.
  3. The pipe from the nhani trap/silt chamber should directly go to the leach pit.
  4. It is always advisable to lay the pipe underground and from the periphery of the courtyard to avoid accidental damage.

Operation and Maintenance


  1. Cleaning of nhani trap and silt chamber should be done weekly/fortnightly or as required; and
  2. De-sludging of the leach pit should be done once in two years. If a silt chamber is provided and is periodically cleaned then the pit does not require any de-sludging for five to six years or more.

(1A) Leach Pit with Concrete Rings


      Leach pit can also be constructed using prefabricated cement rings. Four rings of 1000 mm dia and 300 mm height with proper honeycombing (5 holes per ring, each hole 40 to 50 mm) can be placed one over another and covered with RCC slab / ferrocement slab/flag stone. Please note that if the diameter of ring is less than 1000 mm the number of rings should be correspondingly increased. e.g. if the diameter is 750 to 825 mm the number of rings should be taken as 5 instead of four. This will naturally increase the depth of pit by say 1 ft. The pipe level should not be changed. It should be about 225 mm from the top.


Excavation of pit

      Since the thickness of rings is 40 to 50 mm the diameter of the pit will be 1.1 m & depth will remain same i.e. 1.2 m

      All other details such as design criteria, connection, site selection etc remain the same as in case of a brick-lined leach pit.










1B) Modified Leach Pit


In some semi-permeable soils the rate of absorption of water is slow. In such soils, changes as detailed below may be needed: Increase the diameter of the pit to be excavated by 600 mm.

  1. Construct the pit as usual. This will leave a gap of 300 mm all around the pit. However, the width of honeycombing holes should be reduced to 20 to 25 mm and number of holes should be increased correspondingly.

2. After the construction is over fill this gap with coarse sand. This sand envelope will facilitate seepage of water in the semi-permeable soil.


2) Kitchen Garden


      This is a simple and easy way of reusing wastewater at the household. If grey water is partially treated to retain solids, oils and grease it can be used for growing vegetables/flowers/fruits for household use.


Significance of a Kitchen Garden


This is the most environment-friendly way of handling wastewater. In one way, it is the utilization of waste and, on the other hand, it saves precious fresh water which otherwise would have been used for the same garden. The nutrients contained in the wastewater also enhances plant growth.


Requirements

  1. Adequate space in the backyard for growing plants
  2. Willingness of the householders to grow vegetation in the backyard

Design Considerations (Refer Figure13 A, B, C)


      The design of a kitchen garden will differ with every house. Careful planning is essential. The following factors should be taken into consideration while planning for a kitchen garden:


  1. Space available for garden
  2. Quantity of grey water available for gardening
  3. Plant species intended to be grown

      It should be seen that the grey water generated at the household level is fully utilized in the area available. If the area is limited, grey wastewater that can support plants in the available area should be used in the garden and the rest should be suitably diverted / treated into other systems such as a leach pit.


Description

      As described in previous paragraphs, the household wastewater must pass through a nhani trap or a P trap to remove impurities. The nhani trap should also be covered with a suitable screen to screen out solids from the wastewater. Additional provision of a silt chamber is also recommended to remove organic and inert matter. This will ensure clean water in the kitchen garden and will eliminate odour nuisance and vector breeding.


      The wastewater passed through nhani trap and silt chamber can either be directly used in the kitchen garden or it can be first stored in a suitable storage tank and used as per the need of the plants.


Operation and Maintenance

  1. Cleaning of the nhani trap and silt chamber should be done weekly/fortnightly or as required;
  2. Deposition of inert matter/organic matter/salts on the soil surface should be removed after harvesting of crops;
  3. In the rainy season, when grey water is not required for gardening, it should be diverted/treated in other systems such as a leach pit to avoid stagnation of water and vector breeding; and
  4. Precaution should also be taken to avoid use of strong detergents and chemicals which may have a detrimental effect on the plants grown. If such chemicals are used, the grey water may not be used to grow tubers (potato, onion, etc.)


Unscientific Kitchen Garden

Scientific Kitchen Garden















Estimate for Kitchen Gardens


1) For Kitchen Garden without Storage Tank


Sr. No. Particulars Quantity required Rate Total Cost
1 Silt Chamber 1
2 Leach Pit (optional) 1
3 Pipe line 3m
4 Manual labor (Plumbing etc) 1 M. D.
5 Total

Note : For cost of leach pit & silt chamber refer respective figures



1) For Kitchen Garden with Storage Tank

Sr. No. Particulars Quantity required Rate Total Cost
1 Silt Chamber 1
2 Leach Pit (optional) 1
3 Storage Tank (Capacity 200 to 500 lit) 1
4 Pipe line 3m
5 Manual labor (Plumbing etc) 1 M. D.
6 Total

Note : For cost of leach pit & silt chamber refer respective figures



1) For Kitchen Garden with Piped Root Zone System (5 meters length & two outlets)

Sr. No. Particulars Quantity required Rate Total Cost
1 Silt Chamber 1
2 Leach Pit (optional) 1
3 PVC pipe (63 mm) 5m
4 PVC pipe (25 / 40 mm) for two outlet 1.2m
5 Valve for one outlet (25/40mm) 2
6 PVC ‘T’ (25/40mm) 2
7 Reducer ‘T’ (63:25/40) 2
8 Pipe line from house to system 3m
9 Gravel 0.6 cum
10 Polythene sheet 1.5 sqm
11 Manual labor (Plumbing, excavation etc) 1.5 M. D.
12 Total

Note : 1. Actual cost can be calculated as per the length required at a particular site
2. For cost of leach pit & silt chamber refer respective figures